Intermediate density marker and a method using such a marker for radiographic examination

ABSTRACT

The present invention provides a partially radiolucent, partially radiopaque marker and a method for using such a marker for radiographic examination. The marker and the disclosed method may be employed for the examination of tissue structures of various densities. The radiographic density and thickness of the marker are selected so that when the marker and an underlying tissue structure are exposed to x-ray radiation of a specified energy, the marker casts a legible shadow without obscuring anatomical detail present in the underlying tissue structure. The invention also provides a system of partially radiolucent, partially radiopaque markers for use in the method of radiographic examination taught by the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/372,835filed Aug. 12, 1999, now U.S. Pat. No. 6,985,558 which is a continuationof application Ser. No. 08/934,121 filed Sep. 19, 1997, now U.S. Pat.No. 6,041,094, which is a continuation of application Ser. No.08/372,658, filed Jan. 13, 1995, now abandoned, which is acontinuation-in-part of application Ser. No. 08/059,201, filed May 7,1993, now U.S. Pat. No. 5,383,233, which is a continuation-in-part ofthe co-pending application Ser. No. 09/372,835, filed Aug. 12, 1999, allof which are expressly herein incorporated by reference as part of thepresent disclosure.

BACKGROUND OF THE INVENTION

This present invention relates generally to the field of a radiographyand, more particularly, to a marker of intermediate density and a methodof radiographic examination using such a marker. The invention furtherrelates to a system of radiographic markers for use in such a method.

To convey pertinent information on radiographic film, radiologists andtechnicians frequently use markers which absorb x-rays and cast a shadowwhen placed within the x-ray exposure field. Such markers are positionedeither directly on a patient undergoing radiographic examination or onthe cassette holding the radiographic film.

For example, right and left markers are routinely used to designate theanatomical orientation of the patient or to identify a particularextremity being examined. Markers are also used in trauma cases tolocalize the trauma site by placing the marker on the skin surface atthe appropriate location prior to x-ray exposure. Further, markers areoften placed on the surface of the examination table or the filmcassette, within the exposure field but outside the image of thepatient, to define the patient's physical orientation in relationship tothe x-ray beam or the film, i.e., erect, prone, supine or decubitus.

In the past, radiographic markers have been constructed of materialshaving a high atomic number such as, for example, lead, mercury, steelor heavy metal salts. When exposed to x-rays, markers having such highdensity completely attenuate the x-ray beam and cast a distinct shadowthat is readily apparent on the x-ray film. The disadvantage here isthat any tissue detail that falls within the shadow of the marker willbe completely obscured. Metallic markers also create problems whenemployed in computerized tomography (CT) applications. CT scanners,which typically operate in the range of 60 to 80 KV, cannot tolerate thepresence of metallic objects in the radiographic field. Such denseobjects cause streaking which degrades the radiographic image. Thisproblem is made more acute by the fact that the marker must havesufficient surface area to be imaged in at least one section radiographed by the scanner. A metallic marker having the required surfacearea would completely disrupt the scanning apparatus.

Accordingly, it has been the practice of those skilled in the art ofgeneral radiography to place such markers on the patient or theradiographic film outside the area of clinical concern. This practiceencourages increasing the size of the x-ray field to ensure that theimage of an important marker appears on the film. Unfortunately, oneresult of such a practice is that the patient's body is exposed topotentially harmful radiation beyond the specific site being examined.In cases where a radiographic examination is conducted and the image ofa marker is not clearly visible within the exposure field, the exam isoften repeated to either enlarge the exposure field or to reposition themarker. Again, the unfortunate result is that the patient is exposed togreater dosage of potentially harmful radiation.

To precisely locate a tissue area of particular concern on theradiographic film following exposure, there are circumstances where amarker is placed on a patient and purposely imaged while overlyinganatomical structures. In such a case, it is the usual practice of thoseskilled in the art to use a small, metal, spherical marker measuring nomore than about 1 to 2 mm in diameter. While the shadow from such asmall marker is less likely to obscure important tissue detail, theshadow may be mistaken for a physiological calcification or an opaqueforeign body. Further, because such a marker is necessarily of smallsize with a correspondingly small shadow, there is an inherentdifficulty in discerning variations in the size and shape of themarker's image on the film. Thus, the information that can be conveyedby markers of this type is limited.

Finally, markers comprising lead, mercury and other toxic heavy metalscannot be deposited in landfills or incinerated and thus present apotential environmental hazard if improperly disposed of. This problemis exacerbated by the fact that during radiographic examinationprocedures markers are often contaminated by body fluids. Whilecontaminated markers can be sterilized, to save time and expense theyare, more typically, discarded after only a single use.

It is therefore an object of the invention to provide a radiographicmarker which may be placed on a tissue structure and imaged withoutobscuring underlying anatomical detail.

It is a further object of the invention to provide such marker which isconstructed from non-toxic material.

It is a still further object of the invention to provide markers of thistype in various sizes and shapes to convey pertinent information onradiographic film.

It is another object of the invention to provide a marker for use in CTapplications which may be placed on a tissue structure and imagedwithout obscuring underlying anatomical detail.

It is yet another object of the invention to provide a method ofradiographic examination using such markers.

SUMMARY OF THE INVENTION

The present invention meets these and other objects by providing apartially radiolucent, partially radiopaque marker for the radiographicexamination of underlying tissue structures. The marker has aradiographic density and thickness selected to permit the marker to bothcast a radiographic shadow and transmit sufficient radiation to imageanatomical detail present in the tissue structure when the marker andthe tissue structure are exposed to an x-ray beam.

In a second aspect, the invention provides a method for radiographicexamination which includes the steps of providing a source of x-rayradiation capable of generating an x-ray beam of sufficient energy toimage a tissue structure exposed to the beam, and positioning theabove-described intermediate density marker between the source of thex-ray radiation and the tissue structure. Once the marker has beenproperly positioned, the marker and the tissue structure are exposed tothe x-ray beam to generate a radiographic image of the structure withthe shadow of the marker superimposed thereon. Since the radiographicdensity and thickness of the marker have been carefully selected to meetthe criteria set forth above, anatomical detail present in the tissuestructure is clearly visible through the shadow cast by the marker.

In another aspect, the present invention provides a system ofintermediate density markers for use in the radiographic examination oftissue structures representing a range of tissue densities. Tissuesrepresenting the range of densities for which the markers comprising thesystem may be used include, for example: breast tissue and other softtissues having a density approximately equal to that of water;intermediate density tissues, such as the bones of the extremities; andvery dense tissues such as the skull, spine and the tissues comprisingthe chest and other thick body parts.

Each of the markers comprising the system has a radiographic density andthickness which permit the marker, upon exposure of the marker and anunderlying tissue structure to x-ray radiation of a specific energy, toboth cast a radiographic shadow and transmit sufficient radiation toimage anatomical detail present in the underlying tissue structure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a radiograph of several rubber collars useful in practicingthe method of the present invention imaged on the surface of amammography phantom.

FIG. 2 is a radiograph of rubber, aluminum and vinyl markers useful inpracticing the method of the present invention imaged on the surface ofthe American College of Radiology quality assurance phantom.

FIG. 3 is a cranio-caudal view of a patient's left breast with a vinylmarker useful in practicing the method of the invention positioned onthe surface thereof.

FIG. 4( a) is a posterior view of a plastic/barium sulfate marker usefulin practicing the method of the present invention radio graphed on thesurface of a patient's foot.

FIG. 4( b) is an oblique view of the marker shown in FIG. 4( a).

FIG. 5 is a radiograph of a plastic/barium sulfate marker useful inpracticing the method of the present invention imaged on the surface ofa patient's skull.

FIG. 6 is a perspective view of a marker useful for practicing themethod of the invention.

FIG. 7 is a radiograph of the marker illustrated in FIG. 6 imaged on thesurface of a CT phantom.

DETAILED DESCRIPTION OF THE INVENTION

The most important factor regarding the detail recorded in aradiographic image is the disparity in light and dark or shadowintensity between adjacent objects. This is called image contrast andrefers to the difference in the optical density between areas or objectsimaged on a detectors, commonly photographic film. As x-rays passthrough a patient, or other objects, they are absorbed, to varyingdegree, by the materials through which they pass. The principaldeterminant of contrast is the radiation attenuation property of eachmaterial, which is a function of the material's elemental and chemicalcomposition. Materials of higher atomic number and density (weight pervolume) have a greater ability to absorb x-rays and reduce or attenuatethe beam.

In the diagnostic x-ray spectrum, for example, bone, rich in calcium,contrasts very well with the surrounding muscle and other soft tissueswhich have a density equivalent to water. Fatty tissue, which is only10% less dense than muscle, is not clearly distinguished from other softtissues because of the lack of a significant attenuation differential.

These same conditions hold for all chemical compounds and mixtures,biological and non-biological.

Image contrast, as recorded radiographically, may be expressedmathematically, for a given spectrum of x-rays, as follows:C=(D2−D1)/D1

Where C=the optical contrast differential between objects D1 and D2 asrecorded on a radiograph.

Where D2=the attenuating ability of object D2.

Where D1=the attenuating ability of an adjacent object or the backgroundattenuation.

If a thin, uniform, moderate attenuator is placed in the x-ray beam, theoverall quantity of x-ray in its path is uniformly and moderatelyreduced. Applied to the contrast equation shown above, this additionaluniform attenuation shows up as an equal quantity in the numerator andthe denominator. Mathematically, then, these additional figures cancelout and, C, representing the contrast differential between D2 and D1 isunchanged.

Utilizing these principles, the present invention provides a partiallyradiolucent, partially radiopaque marker having a uniform density andthickness for use in radiographic examination. Depending on the densityof the particular tissue structure being examined and the correspondingradiation energy required, the density and thickness of the marker areselected so that upon exposure of the marker and an underlying tissuestructure to the radiation, the marker will cast a legible shadow onradiographic film without obscuring radiographic anatomical detailpresent in the underlying tissue structure.

With regard to the range of tissue densities presented in humandiagnostic radiology and the corresponding radiation energies required,those skilled in the art are well aware that soft tissue, such as breasttissue and other tissues having a density approximately that of water,require the application of “soft” radiation, generally in the range offrom about 20 KV to about 40 KV. More dense tissue, such as bone tissuein the extremities, requires radiation in the range of from 40 KV toabout 70 KV. Finally, very dense tissue such as, for example, the skull,spine and the tissue structures comprising the chest, require moreintense radiation in the range of from about 70 KV to about 120 KV.

Accordingly, the present invention encompasses markers of varyingdensity and thickness which may be used for the radiographic examinationof tissues representing the broad range of tissue densities noted above.In general, a marker is designed to absorb from about 2% to about 75% ofthe incident radiation. However, it must be appreciated that theinvention is in no way limited in this regard and that the markers mayabsorb more or less radiation depending on the particular tissue beingimaged and its density. The governing parameters are that the markermust exhibit sufficient radiolucency to provide a discernible image whenexposed to radiation and at the same time exhibit sufficient radiopacityso as not to obscure the underlying tissue detail.

Thus, it is no longer necessary, as with prior art radiographic markersand methods of examination, to completely attenuate the x-ray beam toachieve a consistent and easily discernible image of the marker on thefilm. As explained more fully below in connection with the Figures,there is a broad range of materials of intermediate density which can beused to construct the markers taught by the invention.

Referring now to FIG. 1, a number of rubber O-shaped collars 10 having athickness of from about 1.5 mm to about 2 mm are shown radiographed onthe surface of a Kodak mammography phantom 12. The exposure is at 28 KV,and the collars are equivalent to about 1.2 mm of aluminum, asestablished by the aluminum step wedge 14, which is graduated in 0.2 mmincrements. Markers of this type are useful in imaging soft tissue at aradiation energy ranging from about 20 KV to about 40 KV and areparticularly useful in mammography. As FIG. 1 illustrates, the markershave sufficient radiopacity to cast a discernible image and aresufficiently radiolucent to permit the simulated micro calcifications 16and fine tissue details 18 to be clearly imaged.

FIG. 2 illustrates additional examples of markers useful inradiographing low density tissue structures. The markers are constructedof rubber, aluminum and vinyl and are shown radiographed on the surfaceof the American College of Radiology quality assurance phantom (“ACRphantom 006 294”). The exposure is at 26 KV, and the radiograph was madeusing a photo-timed mammography machine available from General Electric.As is well-known to those skilled in the art, the ACR phantom 20approximates breast tissue density and has simulated calcifications andnylon low density “nodules”.

Markers 22 comprise strips of rubber having a thickness of about 0.2inches and a density approximately equal to 0.115 inches of aluminum.Markers 24 also comprise strips of rubber, and the strips have athickness of 0.1 inches. Markers 26 comprise plastic strips having athickness of about 0.1 inches, and markers 28 are strips of aluminumhaving a thickness of 0.115 inches. Finally, markers 30 are strips ofvinyl 0.1 inches thick.

As FIG. 2 illustrates, markers comprised of these various materials casta legible shadow without obscuring underlying detail when exposed toradiation energies typically employed in radiographing low density softtissue. Generally, rubber markers having a thickness of from about 0.2inches to about 0.4 inches are appropriate for radiographing softtissue. With respect to plastic and aluminum markers, thicknessesranging from 0.1 inches to about 0.2 inches and from about 0.011 inchesto about 0.022 inches, respectively, have been found useful. Vinylmarkers provided in thicknesses ranging from about 0.75 mm to about 1 mmhave been found useful.

The utility of markers constructed from vinyl for radiographing softtissue structures without obscuring underlying detail is furtherillustrated in FIG. 3. FIG. 3 is a cranio-caudal view of a patient'sleft breast 32 with a vinyl marker 34 positioned on the surface thereofto mark the location of scar tissue. As FIG. 3 illustrates, theanatomical detail in the breast tissue directly underlying the anteriormarker 34 is clearly visible on the radiograph.

FIG. 4 illustrates posterior and oblique views of a plastic/bariumsulfate marker useful for radiographing more dense tissues at energylevels from about 40 to about 70 KV. The marker 36 is plasticimpregnated with 40% barium, and it is shown imaged on the surface of apatient's foot 38. Note that the anatomical detail in the bone 40underlying the marker 36 is clearly visible.

Markers constructed of barium impregnated plastic have been found to beparticularly useful in practicing the present invention because thedensity of the markers can be easily and precisely selected by varyingthe barium content of the plastic. Thus, an entire selection or systemof barium impregnated plastic markers is provided for the entire rangeof tissue densities and exposure energies typically encountered by thoseskilled in the art.

FIG. 5 shows a plastic/barium sulfate marker useful for radiographingvery dense tissues, such as the bones of the skull, face and hip, atenergy levels from about 70 to about 120 KV. The marker 42 is plasticimpregnated with 40% barium and is cut from sheets measuring between0.015-0.02 inches in thickness. The marker 42 is shown imaged on thesurface of a patient's skull 44, and the radiograph shows that themarker images well at one to two layers of the sheet material(0.015-0.040 inches). The marker is most useful at a thickness of about0.030-0.040 inches where it has sufficient density to be clearly visibleon the radiograph, and yet permit sufficient penetration of theradiation to provide imaging of the diagnostic object contrast.

FIG. 6 illustrates an intermediate density marker 46 useful in CTscanning applications. The marker 46 is shown imaged on the surface of aCT phantom 47 in FIG. 7. The CT phantom 47 is available from the GeneralElectric Corporation under the designation GE #46-241-85-2GI. The markercomprises a vinyl strip 48 having a plurality of apertures 50 formedtherein. When the marker is imaged together with an associated tissuestructure, the apertures indicate, as illustrated in FIG. 7, whether themarker is in the axial plane and also indicates whether the CT cut ishigh, through the middle or low with an accuracy of plus or minus about1 mm. In addition, the apertures provide a site for needle insertion.

The strip 48 is preferably from about 1 mm to about 4 mm thickness, fromabout 1 cm to about 4 cm in width and from about 5 cm to about 20 cm inlength. In the illustrated embodiment, the strip 48 is about 2 mm thick,about 2 cm wide and about 8 cm long. The apertures 50 are about 1 cm indiameter and are space about 0.5 cm apart. As those skilled in the artwill appreciate, the length, width and thickness of the strip may bevaried as long as it is large enough to image clearly.

Markers constructed from a number of different materials and suitablefor radiographing tissues representing a broad range of densities havebeen illustrated above. Those skilled in the art will recognize,however, that there are a great number of other marker configurations aswell as other materials, either alone or in combination, which are of anintermediate density and which could just as easily be used to practicethe present invention.

As noted above, the density and thickness of the marker is selectedbased on the density of the tissue being examined and the energy of theradiation being applied. In general markers are designed to absorb fromabout 2% to about 75% of the incident radiation. For example, in certainCT applications, the marker is constructed to absorb as little as fromabout 2% to about 5% of the incident radiation. Again, the governingparameters are that the marker must absorb enough radiation to cast alegible shadow without obscuring underlying anatomical detail.

Additional factors that influence the ultimate choice of the materialfrom which the marker is constructed include the cost of the material,its toxicity and the ease with which the marker can be manufactured. Itmust also be emphasized again that the selected material must be of auniform density so that the radiographic image of the marker has auniform, translucent appearance without visible irregularities,conflicting striations or granulations which might obscure theresolution of anatomical detail.

The present invention further contemplates a system of partiallyradiolucent, partially radiopaque markers which are provided in a rangeof densities and thickness so that selected markers comprising thesystem may be used in radiographing tissue structures representing thevarious tissue densities enumerated above at the corresponding radiationenergy range. In addition, the markers are provided in a variety ofshapes to enhance the information communicated by the markers. In thisregard, the markers may be shaped in accordance with a universallanguage, wherein each marker conveys a specific, universally acceptedmessage. Such an enhancement of the information visibly communicated bythe markers will expedite film interpretation and improve accuracy. Forexample, markers provided in the shape of an arrow, a triangle, theuniversal symbol of danger, or a standard medical cross could beprovided to indicate an area of clinical concern on the film.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made without departing from thespirit and scope of the invention. Accordingly, it is to be understoodthat the present invention has been described by way of example and notby limitations.

1. A non-toxic marker for mammographic examination of breast tissue thatmay anatomical detail present in the tissue, wherein a source of x-rayradiation is provided for generating radiation at a predetermined energylevel suitable for imaging breast tissue, the marker is positioned onthe breast tissue, and the marker and breast tissue are exposed to thex-ray radiation at the predetermined energy level to generate aradiographic image of the tissue having the shadow of the markersuperimposed thereon, the marker comprising: a first outer surface forcontacting a patient's breast tissue; a second outer surface located onan opposite side of the marker relative to the first outer surface; andat least one non-toxic partially radiolucent, partially radiopaquematerial located therebetween, said at least one non-toxic partiallyradiolucednt, partially radiopaque material defining a density andthickness based on a density of breast tissue and the predeterminedenergy level of the radiation provided which absorbs from about 20% toabout 75% of the incident radiation and, in turn, generates aradiographic image of the tissue having the shadow of the markersuperimposed thereon with any anatomical detail present in the tissueclearly visible through the radiographic shadow projected by the maker.2. A mammographic marker as defined in claim 1, wherein the non-toxicpartially radiopaque, partially radiolucent material is selected fromthe group including: (i) rubber having a thickness of less than about0.4 inches; (ii) plastic having a thickness of less than about 0.2inches; (iii) vinyl having a thickness of less than about 1 mm; and (iv)aluminum having a thickness of less than about 0.022 inches.
 3. Amammographic marker as defined in claim 2, wherein the non-toxicpartially radiopaque, partially radiolucent material is selected fromthe group including: (i) rubber having a thickness within the range of0.2 to about 0.4 inches; (ii) plastic having a thickness within therange of about 0.1 to about 0.2 inches; (iii) vinyl having a thicknesswithin the range of about 0.75 mm about 1 mm; and (iv) aluminum having athickness within the range of about 0.011 to about 0.022 inches.
 4. Amammographic marker as defined in claim 2, wherein the plastic isimpregnated with a metal.
 5. A mammographic marker as defined in claim1, wherein the non-toxic partially radiopaque, partially radiolucentmaterial includes metal and plastic having an overall thickness of about0.015 to about 0.04 inches.
 6. A mammographic marker as defined in claim5, wherein the non-toxic partially radiopaque, partially radiolucentmaterial includes metal and plastic having an overall thickness of about0.04 inches.
 7. A mammographic marker as defined in claim 5, wherein theplastic is impregnated with the metal.
 8. A mammographic marker asdefined in claim 7, wherein the plastic is impregnated with barium.
 9. Aplurality of mammographic markers as defined in claim 1, including: afirst marker defining the shape of a circle; a second marker definingthe shape of a triangle; and a third marker defining the shape of astraight line.
 10. A plurality of mammographie markers as defined inclaim 9, further including; a fourth marker defining the shape of cross;and a fifth marker defining the shape of an arrow.
 11. A method ofmammographic examination of breast tissue that may have anatomicaldetail present in the tissue, comprising the steps of: providing asource of x-ray radiation having a predetermined energy level capable ofgenerating radiation suitable for imaging breast tissue; providing anon-toxic partially radiolucent, partially radiopaque marker having aradiographic density and thickness which, based on the density of abreast tissue and the predetermined energy level of the radiationprovided, absorbs from about 2% to about 75% of the incident radiationand permits the marker to both project a radiographic shadow andtransmit sufficient radiation to image anatomical detail present inbreast tissue when the marker and the tissue are exposed to thepredetermined level of x-ray radiation during mammographic examination;positioning the marker on the breast tissue; and exposing the marker andthe tissue to the x-ray radiation at the predetermined energy level, andgenerating a radiographic image of the tissue having the shadow of themarker superimposed thereon with any anatomical detail present in thetissue clearly visible through the radiographic shadow projected by themarker.
 12. A method of mammographic examination as defined in claim 11,further compromising the steps of adhesively securing the marker to apatient's breast tissue, and transmitting radiation within the range ofabout 20 kV to about 40 kV through the marker and the tissue.
 13. Amethod of mammographic examination as defined in claim 11, wherein themarker is selected from the group including: (i) a marker comprisingrubber having a thickness of less than about 0.4 inches; (ii) a markercomprising plastic having a thickness of less than about 0.2 inches;(iii) a marker comprising vinyl having a thickness of less than about 1mm; and (iv) a marker comprising aluminum having a thickness of lessthan about 0.022 inches.
 14. A mammographic marker as defined in claim1, wherein the non-toxic partially radiolucent, partially radiopaquematerial is of a uniform density such that the radiographic image of themarker has a uniform, translucent appearance without visibleirregularities, conflicting striations or granulations which mightobscure the resolution of underlying anatomical detail.
 15. A method ofmammographic examination as defined in claim 11, further comprisingproviding a marker having a non-toxic partially radiolucent, partiallyradiopaque material that is of uniform density, and generating amammographic image of the marker having a uniform, translucentappearance without visible irregularities, conflicting striations orgranulation obscuring the resolution of any underlying anatomical detailpresent in the tissue.
 16. A non-toxic marker for radiographicexamination of tissue that may have anatomical detail present in thetissue, wherein a source of x-ray radiation is provided for generatingradiation at a predetermined energy level suitable for imaging tissuehaving a predetermined tissue density, the marker is positioned on thetissue, and the marker and tissue are exposed to the x-ray radiation atthe predetermined energy level to generate a radiographic image of thetissue having the shadow of the marker superimposed thereon, the markercomprising: a first outer surface for contacting a patient's skin; asecond outer surface located on an opposite side of the marker relativeto the first outer surface; and at least one non-toxic partiallyradiolucent, partially radiopaque material located between the first andsecond outer surfaces, said at least one non-toxic partiallyradiolucent, partially radiopaque material defining a density andthickness based on the predetermined tissue density and thepredetermined energy level of the radiation provided which absorbs fromabout 2% to about 75% of the incident radiation and, in turn, generatesa radiographic image of the tissue having the shadow of the markersuperimposed thereon with anatomical detail present in the tissueclearly visible through the radiographic shadow projected by the marker,and said at least one non-toxic partially radiolucent, partiallyradiopaque material is of a uniform density such that the radiographicimage of the marker has a uniform, translucent appearance withoutvisible irregularities, conflicting striations or granulations whichmight obscure the resolution of underlying anatomical detail.
 17. Aradiographic marker as defined in claim 16, wherein the non-toxicpartially radiopaque, partially radiolucent material is selected fromthe group including: (i) rubber having a thickness of less tan about 0.4inches; (ii) plastic having a thickness of less than about 0.2 inches;(iii) vinyl having a thickness of less than about 1 mm; and (iv)aluminum having a thickness of less than about 0.022 inches.
 18. Aradiographic marker as defined in claim 17, wherein the plastic isimpregnated with a metal.
 19. A plurality of radiographic markers asdefined in claim 16, wherein each of the plurality of markers defines arespective radiographic density different than the other markers andcorresponds to a respective tissue density for imaging tissue havingthat density, and including (i) a first marker defining a radiographicdensity approximately equal to the density of water for imagingrelatively soft tissue; (ii) a second marker defining a radiographicdensity approximately equal to the density of the bones of a patient'sextremities for imaging relatively intermediate density tissue; and(iii) a third marker defining a radiographic density approximately equalto the density of at least one of a skull, spine and chest of a patientfor imaging relatively dense tissue.
 20. A radiographic marker asdefined in claim 16, wherein the marker is a mammographic marker formammographic examination of breast tissue, the marker is adhesivelyattachable to a patient's breast tissue for positioning the marker onthe breast tissue, and the predetermined level of the x-radiation iswithin the range of about 20 kV to about 40 kV.
 21. A method ofradiographic examination of tissue that may have anatomical detailpresent in the tissue, comprising the steps of: providing a source ofx-ray radiation having a predetermined energy level capable ofgenerating radiation suitable for imaging a predetermined tissuedensity; providing a non-toxic partially radiolucent, partiallyradiopaque marker having a radiographic density and thickness whichpermit the marker to both project a radiographic shadow and transmitsufficient radiation to image anatomical detail present in tissue havingthe predetermined tissue density, when the marker and the tissue areexposed to the predetermined level of x-ray radiation duringradiographic examination, said providing step including providing amarker having a density and thickness based on the predetermined tissuedensity and the predetermined energy level of the radiation providedthat absorbs from about 2% to about 75% of the incident radiation, andthat includes a non-toxic partially radiolucent, partially radiopaquematerial of a uniform density such that the radiographic image of themarker has a uniform, translucent appearance without visibleirregularities, conflicting striations or granulations which mightobscure the resolution of underlying anatomical detail; positioning themarker on the tissue having the predetermined tissue density; andexposing the marker and the tissue to the x-ray radiation at thepredetermined energy level, and generating a radiographic image of thetissue having the shadow of the marker superimposed thereon with anyanatomical detail present in the tissue clearly visible through theradiographic shadow projected by the marker.
 22. A plurality ofmammnographic markers as defined in claim 9, wherein each markerincludes an adhesive and is adhesively attachable to the breast tissue.23. A plurality of mammographic markers as defined in claim 1, whereineach marker defines a shape that conveys a specific message, including afirst marker defining a circular shape, a second marker defining atriangular shape, and a third marker defining a linear shape.
 24. Amethod of mammographic examination as defined in claim 11, furthercomprising the steps of providing a plurality of said markers inpredetermined shapes enhancing the information communicated by themarkers, including: (a) a first marker defining the shape of a circle;(b) a second marker defining the shape of a triangle; and (c) a thirdmarker defining the shape of a line.
 25. A method of mammnographicexamination as defined in claim 24, wherein said step of providing aplurality of said markers in predetermined shapes further includesproviding: a fourth marker defining the shape of an arrow.
 26. A methodof mammographic examination as defined in claim 11, further comprisingthe steps of adhesively securing the marker to a patient's breasttissue.